In the continuous casting of molten material, particularly metals and alloys such as steel, the apparatus for this purpose includes a tundish for receiving the molten material to be cast. The tundish has a refractory, horizontally disposed nozzle through which the molten metal leaves the tundish for casting. The nozzle at the end opposite that connected to the tundish is connected to a flow-through continuous casting mold. The continuous casting mold is made of a heat-conducting material, usually copper, and provision is made for circulation of liquid, usually water, to cool the mold. As the molten metal enters the mold and contacts the cooled, interior mold surfaces, it is solidified to form a solidified skin of the molten metal with the interior portion remaining in the molten condition. The thickness of the solidified skin increases progressively along the length of the mold. As this partially solidified or embryo casting leaves the mold, the skin is sufficiently thick to prevent the breakout of molten metal. Thereafter, the embryo casting is progressively cooled and eventually complete solidification of the casting is achieved.
The refractory material of the nozzle which is in contact with the molten metal leaving the tundish is at extremely high temperatures. In contrast, the mold abutting the refractory nozzle is at significantly lower temperatures. Consequently, it is conventional practice to place on the interior of the mold a break ring to serve as a transition material between the refractory of the nozzle and the heat-conducting material of the cooled mold. The break ring functions to define the point at which the shell of the casting begins to form when the molten metal initially enters the mold. It prevents solification and hang-up of the metal at the end of the nozzle at the interface of the nozzle and mold. More specifically in this regard, as is well known, relative oscillation or vibration of the mold and casting is provided longitudinally to facilitate withdrawal of the partially solidified casting from the mold. If metal enters and solidifies at the interface or connection between the nozzle and mold a solidified metal projection, commonly termed a "fin", is formed. This during withdrawal results in surface irregularities on the casting skin, which can cause cracking with resulting molten metal break-out. In addition, the break ring prevents the molten metal from freezing within the pores of the refractory material adjacent the continuous casting mold. In summary, the break ring prevents damage to the newly formed solidified casting skin at the entry end of the mold. For this purpose, and particularly in the continuous casting of molten alloys such as steel, the break ring must have chemical resistance to the steel, high resistance to thermal shock, low thermal conductivity, high resistance to wear and erosion and accurately conform to the surface on which it is mounted. To meet these requirements, the break ring is conventionally constructed from refractory oxides or nitrides, such as boron nitride, silicon nitride and zirconia, and is machined to the proper contour to achieve the required accurate mounting.
The continuous casting mold in typical steel casting operations is of a generally round and/or rectangular configuration to permit the casting of slabs or billets which are subsequently reduced to flat-rolled sheet and strip or bars, respectively. Consequently, the interior cross section of the continuous casting mold must conform to this desired configuration. Each mold, however, due to inaccuracies in construction will vary somewhat in dimension. Therefore, the break ring requires machining to very close tolerances to mate with the interior of the continuous casting mold. If the break ring is not accurately dimensioned with respect to the mold interior onto which it is mounted, this will result in the molten metal propagating between the connection of the break ring and the mold interface surface.